1. Field of the Invention
The present invention relates to a photopolymerizable inkjet ink, an ink cartridge, and an inkjet recording device.
2. Description of the Related Art
Inkjet printing has been conventionally widely known. The characteristics thereof are high use efficiency of materials, as it is a material addition printing type, low cost, and energy-saving and resource-saving production. Therefore, there is a great expectation for the inkjet printing as a production technique, which can reduce environmental load. Moreover, the inkjet printing can be performed on various media, as it is a non-contact printing system. Therefore, applications of the inkjet printing to various fields are expected.
Recently, in the inkjet system, a photopolymerizable inkjet technique using an ink containing a photopolymerization initiator and a polymerizable compound has been developed. In a typical system of this technique, an ink is fixed on a medium by applying UV light from a UV lamp just after printing to cure the ink. Therefore, the speed for drying and fixing is fast compared to an aqueous ink or a solvent-based ink, and high speed printing can be achieved using the photopolymerizable inkjet ink.
A main stream of a photopolymerizable inkjet ink used in the aforementioned system is a non-aqueous 100%-solid ink containing a photopolymerization initiator, which generates free radicals upon application of UV light, and an acryl-based compound. Therefore, this ink can be applied for a material of non-absorption nature, and does not use a large amount of an organic solvent as in a solvent-based ink, and thus safety in printing scene is maintained or environmental load can be reduced. For these reasons, importance of the photopolymerizable inkjet ink is increased even more.
Meanwhile, a majority of polymerizable compounds used in conventional photopolymerizable inkjet inks are toxic. Most of (meth)acrylic acid esters, which are readily available with low cost, have high toxicity in terms of skin sensitivity, which causes allergy reactions with skin upon contact with them. Conventional art has not yet provided any solution to this problem.
Based on the studies conducted so far, the present inventors have found a few (meth)acryl amides, which have no problem in skin sensitization, through a resulting ink has not excellent curing ability. As for one of examples thereof, proposed is a photopolymerizable inkjet ink containing methacrylate as a main component, as methacrylate is lower in toxicity in terms of skin sensitization than acrylate (see Japanese Patent Application Laid-Open (JP-A) Nos. 2012-251123 and 2012-140593).
Moreover, curability of a photopolymerizable inkjet ink also largely depends on reactivity of a photopolymerization initiator to be contained. Therefore, disclosed is a photopolymerizable inkjet ink containing a photopolymerization initiator, which can give sufficient curability and improve odor, through there is a problem in safety (see JP-A No. 2005-533156.
Curability can be improved by increasing an amount of a photopolymerization initiator. However, use of an excessive amount of a photopolymerization initiator is not preferable, because the photopolymerization initiator may cause odor after curing or yellowing. Moreover, curability can be improved by increasing an exposure dose of UV light. However, it may adversely affect the production process, as radiation duration is extended, or a plurality of irradiation devices is used. Therefore, there is a need for a photopolymerizable inkjet ink, which can achieve sufficient curability with a small amount of a photopolymerization initiator, and has excellent efficiency.
When a conventional photopolymerization initiator is used, moreover, there are various problems, other than curability. Especially, odor caused by a decomposed product of a photopolymerization initiator after applying UV light, and yellowing of a coating film are recently very serious problems in the production site. These may not only deteriorate the properties of a coat, but also impairs working environment.
Therefore, improvements in the aforementioned odor and yellowing of the coating film are urgent tasks.